1) Field of the Invention The invention relates to supported catalysts and their use in the gas-phase oxidation of hydrocarbons.
2) Background Art
Supported catalysts for the gas-phase oxidation of hydrocarbons to the corresponding oxidation products, such as, for example, carboxylic acids, carboxylic anhydrides or aldehydes, have long been known. A typical example of use of such catalysts is the preparation of phthalic anhydride from o-xylene or naphthalene, maleic anhydride from benzene or butane, formaldehyde from methanol, or acrylic acid or acrolein from propene. Recently, the preparation of acetic acid by the oxidation of ethane or butane or butene and butane/butene mixtures using supported catalysts has also been described. Common to all these preparation processes is the fact that the reactions are highly exothermic. For this reason, virtually all such processes are carried out in so-called tubular reactors. Here, the tubes are filled with a catalyst and removal of the resulting heat of reaction (cooling) is usually effected by means of a salt melt with which the reaction tubes inside the reactor are surrounded. Depending on the temperature range, the cooling can alternatively also be effected by means of steam, superheated water or other heat-transfer liquids.
The catalysts used are predominantly supported catalysts which, as a rule, consist of an inert support, for example of annular or spherical shape, on which the actual catalytically active material is applied. In the case of the phthalic anhydride and acetic acid catalysts, such catalytically active materials predominantly consist of, for example, TiO2 in the anatase form and V2O5. For improving the control of the activity and improving the selectivity, additionally activating or damping additives, for example oxides of elements of the subgroups of the Periodic Table, alkali metal compounds and/or, in small amounts, promoters are frequently mixed as dopants with the catalytically active material. In the case of catalysts for the preparation of maleic anhydride, the catalytically active material consists of, for example, vanadyl pyrophosphate.
In the preparation of the supported catalysts, in general, suspensions of catalyst powder and liquids (water, organic solvents) or solutions or suspensions of the individual catalyst components, if required with the addition of binder for improving the adhesion of the active components on the support, are sprayed onto the supports.
Furthermore, EP-B 714700 (U.S. Pat. No. 5,677,261) discloses the application of dry powder to moistened supports.
The supports usually used are regularly shaped, mechanically stable bodies, such as spheres, rings, half-rings or saddles. The size of the supports is determined predominantly by the reactor dimensions, especially by the internal diameter of the individual reaction tubes. Support materials used are, for example, steatite, duranite, earthenware, silica, silicon carbide, aluminates, metals and metal alloys.
In choosing the shape of the support and its dimensions, in particular, the associated pressure drop plays an important role. A small pressure drop across the catalyst bed can mean a considerable energy saving, for example in the case of the fan energy.
A further criterion is that the support materials can be produced as economically as possible. Rings and spheres have become established in industry, annular supports being increasingly used owing to the smaller pressure drop present in the case of the rings.
In the past, there has been no lack of attempts, by varying the ring shape, to find support materials which have an optimum pressure drop (i.e., as low a pressure drop as possible) and carry as much active material as possible, but without adversely affecting the other performance data such as selectivity, stability, productivity, etc.
DE-A 344 5289 (U.S. Pat. No. 4,656,157) describes, for example, an annular support which differs from xe2x80x9cnormal ringsxe2x80x9d in that its end faces are rounded. This annular support is said to permit more uniform filling of the reaction tubes and hence a more uniform course of the reaction. Nothing is stated with regard to the pressure build-up.
EP-B 552287 discloses an unsupported catalyst for the preparation of maleic anhydride, which catalyst consists of a solid geometric shape in which at least one cavity is arranged in its outer surface. The examples describe exclusively shapes in which the cavities are arranged in the outer surfaces and not in the end faces. The aim of these shapes is to obtain as large a surface area of the unsupported catalysts as possible. The shapes shown can be realized technically only with great effort and at high costs.
EP-A 220933 discloses an unsupported catalyst for use in catalytic processes, which catalyst has a xe2x80x9cfour-wingedxe2x80x9d shape. This shape is obtained by extruding the catalyst material. Owing to its specific shape, the catalyst has better physical properties with respect to breaking strength and pressure build-up.
GB-A 2193907 describes a catalyst of cylindrical shape whose outer surface is provided, in the longitudinal direction, with ribs which are dimensioned and arranged in such a way that the individual catalyst body cannot interlock.
U.S. Pat. No. 4,328,130 likewise describes a catalyst shape in which a plurality of channels and ribs run along the longitudinal direction of a cylinder, the recesses being narrower than the ribs in order to avoid interlocking.
EP-A 004079 (U.S. Pat. No. 4,370,492 and U.S. Pat. No. 4,370,261) discloses catalyst shapes which are extruded sections having a star-shaped cross-section or are ribbed extrudates.
U.S. Pat. No. 3,966,644 describes catalyst shapes comprising extrudates in the form of a plurality of cylinders joined parallel to one another.
Those forms for catalyst supports which are described in the prior art and permit reduced pressure build-up all have very complicated shapes. The preparation generally entails high costs and is therefore uneconomical on an industrial scale.
Many of these complicated shapes, which are obtainable in particular by extrusion, are unsuitable for coating by means of active catalyst material owing to their surface and therefore can be used only as unsupported catalyst.
The object was therefore to provide catalyst supports which, on the one hand, have a smaller pressure build-up than conventional rings or spheres in the reactor but, on the other hand, have a highly geometric and hence coatable surface which however is as simple as possible. In addition, these support shapes should be simple and economical to prepare and should differ geometrically from the industrially used supports only to an insignificant extent, in order to be able to use them without problems in existing oxidation plants and processes. Furthermore, the supported catalysts to be developed should have just as good a stability as those known from the prior art, should be capable of being introduced into the reaction tubes by means of the known filling machines and should permit a uniform coat thickness when coated with the active material.
The invention relates to supported catalysts consisting of an active material on an inert support in the shape of rings, wherein the rings have one or more notches in the upper and/or lower flat side of the ring.